This invention relates generally to solid-state relays and relay controllers. More particularly, the invention relates to a solid-state relay controller for converting analog electrical signals into digital signals of a variable duty cycle proportional to the analog signal for directly energizing the solid-state relay to effect power proportioning control.
The solid-state relay is a relatively new electronic device for problem solving. It can handle large amounts of power, 40 amperes at 240 volts, for example. Many of today's solid-state relays are small, on the order of about 2".times.11/2".times.3/4", and can be turned on and off by a digital signal (on/off) signal at logic levels of between nominally 3 and 32 volts DC. Most solid-state relays are optically isolated and contain zero crossing firing circuits which cut down on radio frequency interference (RFI). Solid-state relays have been used in the past to turn pumps and heaters on and off, but by themselves lack the ability to regulate the quantity of power delivered to a load to effect power proportioning. Heretofore, it has not been possible to control solid-state relays using analog signals, such as of the type produced by instrumentation and control circuits, and by computer-controlled digital to analog converter circuits.
Circuits utilizing silicon controlled rectifiers (SCR) are known for controlling the power delivered to a load in response to analog signals. The conventional SCR-controlled light dimmer circuit is an example. Such circuits are commonly referred to as phase-fired circuits and operate by controlling the point on the input AC waveform where conduction begins to reguate the power delivered to the load. Typically these circuits are synced to the AC waveform.
Conventional dimmer-type circuits employing SCR's or the like are not often suitable in controlling heavy power consuming equipment particularly in the laboratory, where radio frequency interference can upset delicate experiments. High power phase-fired controllers of the conventional types require expensive, heavy-duty SCR's, and SCR circuits have traditionally been plagued with radio interference problems. Radio interference often results when the SCR device applies power to the load at or near the peak of the AC waveform. To minimize this interference, considerable effort is made to design circuits which switch near the zero crossing of the AC waveform. Often expensive RF filters and shielding must also be employed.
The present invention overcomes the above shortcomings of conventional phase-fired SCR circuits by providing a means for utilizing solid-state relays in a power proportioning mode never before realized. The invention is capable of controlling the power delivered to a high wattage load, such as single phase and multiphase high wattage heaters, in response to a wide range of different analog input signals. The invention is well adapted to providing power proportioning control in response to 0-10 V signals, 0-5 V signals, 4-20 mA signals and 10-50 mA signals. Thus, the invention is well adapted for computer control applications where a computer-controlled D to A converter provides the analog input signal for controlling a load in either open loop or closed loop operation. In addition, the invention is capable of being used in place of a conventional wirewound variac, at a considerable weight savings, size savings and cost savings.
In accordance with the invention, a power proportioning controller is provided. The controller comprises a means for receiving an analog input signal, in the form of an optical isolator with selectible current scaling resistors. Also provided is a means for generating a variable duty cycle digital signal and a means responsive to the analog input signal receiving means for controlling the duty cycle of the digital signal in proportion to the analog input signal. A solid-state relay is energized by the variable duty cycle signal and the relay delivers proportioned power to a load based on the analog input signal.
Further in accordance with the invention, the components which comprise the power proportioning controller are disposed on a circuit board which bolts directly onto and establishes electrical connection with the terminals of the solid-state relay. The controller circuit further comprises a fully regulated AC to DC power supply circuit which derives all necessary power for operating the controller circuit directly from the power output terminals of the solid-state relay device. The resulting power proportioning controller is quite compact, the entire assembly being only slightly larger than the solid-state relay by itself. The controller circuit has remarkable fan out and is capable of controlling at least ten separate solid-state relays simultaneously. This gives the invention the ability to easily control all phases of a three-phase load simultaneously. The invention is further provided with a powerstat circuit which may be used to modify the sensitivity range of the controller to provide a convenient method for adjusting the range in which the load operates. In an electric heater, for example, the output of the heater can be readily adjusted to suit the heating load requirements by simply adjusting the powerstat circuit.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.